A fuel cell is a kind of power generator capable of generating electric energy by electrochemically oxidizing a fuel such as hydrogen or methanol, and an intense interest has been shown towards the fuel cell, as a clean energy supply source, recently. Particularly, it is expected that a polymer electrolyte fuel cell is widely used as a distributed power generation facility of comparatively small scale, and a power generator of mobile bodies such as automobile and marine vessel, because of such high standard operation temperature as about 100° C. and high energy density. Also, an intense interest has been shown towards the polymer electrolyte fuel cell as a power supply of a portable mobile equipment and a portable device, and it is expected to install the polymer electrolyte fuel cell in a cellular phone and a personal computer in place of a secondary cell such as nickel-hydrogen cell or lithium ion cell.
In the polymer electrolyte fuel cell, an intense interest has been shown towards a direct methanol type fuel cell in which metal is directly supplied as a fuel (hereinafter, referred to as DMFC), in addition to a conventional polymer electrolyte fuel cell in which a hydrogen gas is used as a fuel (hereinafter, referred to as PEFC). DMFC has such an advantage that the fuel is liquid and no reformer is used and, therefore, energy density increases and an operating time per one filling of the portable device increases.
In the fuel cell, anode and cathode electrodes in which the reaction capable of generating electricity, and a polymer electrolyte membrane serving as a proton conductor between an anode and a cathode form a membrane electrode assembly (hereinafter abbreviated to MEA) and a cell comprising separators and MEA-interposed between the separators is formed as a unit.
As required properties of the polymer electrolyte membrane, high proton conductivity is exemplified, first. Also, since the polymer electrolyte membrane functions as a barrier which prevents a direct reaction between a fuel and oxygen, low permeability is required to the fuel. Particularly, in a polymer electrolyte membrane for DMFC in which an organic solvent such as methanol is used as the fuel, methanol penetration is referred to as methanol crossover (hereinafter sometimes abbreviated to MCO) and causes a problem such as decrease in cell output and energy efficiency. As other required properties, resistance to solvents is also an important property in DMFC in which a high concentration fuel such as methanol is used, in view of long-term durability against the high concentration fuel. Other required properties include chemical stability for enduring a strong atmosphere during operation of a fuel cell, and mechanical strength and physical durability for enduring thinning and repetition of swelling and drying.
As the material of the polymer electrolyte membrane, NAFION® (manufactured by DuPont Co.) as a perfluorosulfonic acid-based polymer has widely been used. Although NAFION has nearly good balanced properties suited for use as the polymer electrolyte membrane, further improvement is required as the cell is popularly put into practical use. NAFION® is very expensive because it is prepared through multi-stage synthesis, and also has a problem that fuel crossover is large because a cluster structure is formed. Also, there were problems that mechanical strength and physical durability of the membrane formed by swelling and drying are lost because of poor resistance to hot water and poor resistance to hot methanol, and that it cannot be used at high temperature because of low softening point, and a problem such as waste disposal after use and a problem that it is difficult to recycle the material.
To solve these problems, some studies on a polymer electrolyte material containing a hydrocarbon-based polymer of a nonperfluoro-based polymer as a base have been made. As a polymer skeleton, particularly intensive study on an aromatic polyether ketone and an aromatic polyethersulfone has been made in view of heat resistance and chemical stability.
For example, there have been proposed a sulfonated compound of a slight soluble aromatic polyetherether ketone (see, for example, non-patent document 1), polysulfone in a narrow sense as an aromatic polyethersulfone (hereinafter sometimes abbreviated to PSF) and a sulfonated compound of polyethersulfone (hereinafter sometimes abbreviated to PES) in a narrow sense (see, for example, non-patent document 2). However, there was a problem that, when the content of an ionic group increases so as to enhance proton conductivity, the membrane thus formed swells, resulting in large crossover of the fuel such as methanol. Also, there was a problem that the membrane thus formed is insufficient in mechanical strength and physical durability because of low cohesive force of polymer molecular chains.
Also, a sulfonated compound of an aromatic polyether ketone (hereinafter abbreviated to PEK) was proposed (see, for example, patent document 1 and 2). However, there was a problem that, because of its high crystallinity, in case of the composition of low density of a sulfonic acid group, the remained crystal is insoluble in a solvent, resulting in poor processability. To the contrary, when the density of the sulfonic acid group increases so as to enhance processability, the polymer is not crystalline and drastically swells in water and, therefore, the membrane thus formed shows large fuel crossover and is insufficient in strength.
As a method of controlling an amount of the sulfonic acid group in an aromatic polyethersulfone-based material, there is reported a sulfonated aromatic polyethersulfone in which a monomer having a sulfonic acid group introduced therein is polymerized and an amount of a sulfonic acid group is controlled (see, for example, patent document 3). However, a problem such as swelling of a membrane formed at high temperature and high humidity is not solved by this technique. Particularly, in an aqueous solution of a fuel such as methanol, or in case of the composition of high density of a sulfonic acid group, there is remarkable tendency of swelling of the membrane. In a polymer electrolyte membrane which is inferior in resistance to hot water and resistance to hot methanol, it was difficult to sufficiently control crossover of the fuel such as methanol and to impart mechanical strength and physical durability which can endure a swelling and drying cycle.
As described above, the polymer electrolyte material of the prior art was insufficient in means for improving economical efficiency, processability, proton conductivity, fuel crossover, resistance to solvents, mechanical strength, physical durability and long-term durability, and there has never been obtained an industrially useful polymer electrolyte material for fuel cell.
non-patent document 1: “Polymer”, 1987, vol. 28, 1009 non-patent document 2: Journal of Membrane Science, 83 (1993) 211-220 patent document 1: Japanese Unexamined Patent Publication (Kokai) No. 6-93114 patent document 2: Published Japanese Translation No. 2004-528683 of the PCT Application patent document 3: U.S. Patent No. 2002/0091225.